Modern refrigerators often comprise two or sometimes more compartments to be maintained at different temperatures. Normally, they comprise a freezer compartment for storing deep-frozen food at approx. −18° C. and a fresh food compartment for storing food at approx. +5° C. In the following, such compartments are referred to as freezer and fridge respectively. Sometimes, e.g. at larger household refrigerators, each compartment is cooled by a separate refrigeration apparatus. However, quite often a single refrigeration apparatus is utilized for cooling both the freezer and fridge. This is especially true for smaller household and mobile appliances such as at absorption refrigerators for recreation vehicles and mobile homes. At such refrigerators, the refrigerator apparatus comprises a condenser and an evaporator. Compressor refrigerators further comprise a compressor, whereas absorption refrigerators instead further comprise a boiler and an absorber. The evaporator comprises an evaporator tube for conducting a cooling medium. The evaporator tube is arranged so that it passes inside the compartments.
At absorption refrigerators, the evaporator reaches its lowest evaporation temperature at an uppermost, upstream end of the evaporator tube. Below and downstream and of the upstream end, the evaporation temperature rises gradually when the cooling medium in the tube absorbs heat from the air in the compartments. For this reason, the freezer is normally arranged to be cooled by an upstream portion of the evaporator tube, whereas the fridge is cooled by a tube portion being arranged downstream of the freezer tube portion.
At this type of refrigerators, the air to be cooled is normally circulated through self-circulation inside the respective compartments. Such self-circulation occurs due to a difference in density between cooler and warmer air. When air passes the evaporator, heat is transferred from the air to the cooling medium in the evaporator tube. The temperature of that air thus decreases, whereby its density increases. That recently cooled air thereby falls by influence of the gravity to the lower portion of the compartment. At the lower portion of the compartment and during its movement in the compartment, the air absorbs heat from the food stored in the compartment. When the cold air falls from the evaporator, a low pressure is created, whereby warmer air is drawn from the upper portion of the compartment to the evaporator. Thus the self-circulation in the compartment continues as long as the evaporator is kept at a lower temperature than other surfaces inside the compartment, such as the surfaces of the stored foodstuff.
For enhancing the heat transfer from the air in the compartments to the cooling medium, a heat exchanger may be arranged in heat conducting contact with a portion of the evaporator tube arranged in the respective compartment. The main function of the heat exchanger generally is to enlarge the surface area of the heat conducting material, which is in contact with the air to be cooled and the cooling medium in the evaporator tube. For this purpose the heat exchanger typically comprises a plurality of fins, which are arranged in heat conducting contact with the evaporator tube.
During normal operation of the refrigerator cabinet, humid air enters into the compartments e.g. when the cabinet doors are opened. As the humidity condenses on the cold surfaces inside the compartments, frost is created on these cold surfaces. Such development of frost is particularly severe on the coldest surfaces, i.e. on the evaporator tube and the heat exchanger in the freezer compartment. The formation of frost on the heat exchanger deteriorates the heat transfer from the air to the cooling medium and thereby lowers the cooling power of the compartment. If the refrigerator apparatus is not dimensioned to compensate for such loss in heat transfer, the temperature in the compartment rises, while jeopardizing the condition of the foodstuff stored in the compartment or the maximum possible storage time. In order to solve this problem, modern refrigerators may comprise means for defrosting the heat exchanger at regular intervals. In such case, the defrosting means is normally applied to the heat exchanger in the freezer, but it may also be applied in the fridge.
A major disadvantage with the above-described multi-compartment refrigerators, which utilize a single refrigeration apparatus, is that the temperatures in the different compartments cannot be controlled independently of each other. Since all compartments are cooled by the cooling medium in the same evaporator tube, it is not possible to regulate the evaporation temperature of the medium in the freezer portion of the evaporator without also influencing the evaporation temperature in the fridge portion and vice versa.
The evaporation temperature of the medium is normally controlled by running the refrigeration apparatus intermittently and regulating the length of the run and stop periods. In practice, a temperature-sensing device is arranged in one of the compartments, in which controlled compartment it is considered to be most important to keep the temperature within the preferred interval. Normally this is the fridge. The temperature sensor is connected to means for activating and deactivating the refrigeration apparatus. As soon as the temperature in the controlled compartment rises above a set value, the refrigeration apparatus is activated, whereby the evaporation temperature of the cooling medium is lowered. Thereby, the heat absorbing capability of the medium is increased and more heat is transferred from the air in the controlled compartment to the cooling medium in that portion of the evaporator, which is arranged in the controlled compartment. When the temperature in the controlled compartment has decreased to the desired value, or a value somewhat lower than that, the refrigeration apparatus is stopped. More or less sophisticated control algorithms may be utilized for calculating when to activate and de-activate the refrigeration apparatus in relation the actual temperature in the controlled compartment as well as other parameters, such as the time of the day, the ambient temperature etc. Further more, instead of being run intermittently, some refrigeration apparatuses may be controlled to run with varying cooling power in response to the actual temperature monitored by the sensor.
However, since also the non-controlled compartment is cooled by the same evaporator and refrigeration medium, the temperature in this compartment will vary in relation to the need for cooling the controlled compartment. If e.g. the refrigerator is used in a warm climate and the fridge door is frequently opened, there will be a great need for cooling the fridge and thereby the freezer will also be kept at a low temperature. If however the same refrigerator is used in a colder climate and the fridge is not so fully loaded or the fridge door not so frequently opened, then the freezer temperature will be higher. This phenomenon is naturally most unwanted and it is often perceived as a paradox by the user, concluding that there is something wrong with the refrigerator. The problem is especially articulate in mobile applications where the refrigerator may be used in varying climates.
A further disadvantage related to the above-described, is that it is not possible to run one of the compartments at a temperature other than what was intended by the manufacturer, while running the other compartment as intended. In other words, in a dual-compartment freezer-fridge combination it is not possible to run both compartments as fridges or freezers if that would be desired.
Another problem concerns defrosting of the evaporator and heat exchanger. And for that reason, defrosting of freezer compartments has up to now only been successfully applied to compressor refrigerators. In order to achieve defrosting of the heat exchanger, an electrical heater in the form a resistive film may be applied to the heat exchanger. The defrosting is activated at regular intervals and the refrigeration apparatus is then de-activated, while the resistive film is activated. The heat exchanger is then heated so that the frost formed thereon is melted. When defrosting is completed, the film is de-activated and the refrigeration apparatus re-activated.
A serious problem, which then occurs, is that also the air surrounding the heat exchanger is warmed up by the resistive film during defrosting. Such heating of air causes a reversed convection in the compartment, so that the heated air is distributed in the compartment by reversed self-circulation.
Thereby, a great amount of the heat generated for defrosting is instead used for heating the air in the compartment. This is naturally most unwanted since it reduces the efficiency of the defrosting and prolongs the time needed for defrosting the heat exchanger. Even more serious however, is that the circulation of heated air causes the entire compartment as well as the foodstuff stored therein to be warmed up. Bedsides that such warming up may deteriorate the quality of the foodstuff, it also increases the time and energy needed for bringing the temperature in the compartment back to the desired, after completion of the defrosting cycle.
This constitutes a particularly sever problem when trying to apply defrosting to freezers in absorption refrigerators. The comparatively low cooling capacity of absorption refrigeration apparatuses often makes it difficult to maintain the desired freezer temperature even without the additional heat added by the defrosting heater.